Airblast circuit breaker with improved sealing means

ABSTRACT

Discloses an airblast circuit breaker comprising interrupter tanks at high voltage filled with high-pressure air and provided with terminal bushings filled with a dielectric gas other than air. The tanks are mounted on hollow insulating columns through which air lines of insulating material extend to supply highpressure air to the tanks. A current transformer adjacent the columns has an insulating housing that is pneumatically interconnected with the bushings in the high-voltage region and is filled with dielectric gas of the same type as the bushings to serve as a reservoir for supplying the bushings with dielectric gas. Also discloses special means for mounting and sealing the high-pressure air line with respect to its column and tank.

United States Patent [72] Inventors Robert S. Barton 3,214,544 10/1965 Leeds 200/148 E Bryn Mawr; 3,364,328 l/1968 Manz..... 200/148 X John A. Oppel, Aldan, both of Pa. 3,513,277 /1970 Sciscione 200/148 gr m; 5 1970 FOREIGN PATENTS l e ar. Patented 06.12, 1971 593,230 /1947 Great Britain 200/148 D [73] Assignee General Electric Company Primary ExaminerRobert S. Macon Att0rneys.l Wesley I-Iaubner, William Freedman, F rank L.

Neuhauser, Oscar B. Waddell and Joseph B. Forman [54] AIRBLAST CIRCUIT BREAKER WITH IMPROVED SEALING MEANS l D s c 4 rawmg Flgs ABSTRACT: Discloses an airblast circuit breaker comprising [52] U.S.CI 200/148 R, i t r t r tanks at high voltage filled with high-pressure air 200/148 D, 174/ 14 B" and provided with terminal bushings filled with a dielectric gas [51] lIlLCl .110! 33/82 other than air, The tanks are mounted on hollow insulating [50] Fleid 0f Search ZOO/148 A, columns through which air lines of insulating material extend 148 8 143 R, 150 14 to supply high-pressure air to the tanks. A current transformer 15 3H, 16 BH adjacent the columns has an insulating housing that is pneumatically interconnected with the bushings in the high-voltage [56] References and region and is filled with dielectric gas of the same type as the UNITED STATESPATENTS bushings to serve as a reservoir for supplying the bushings with 2,160,660 5/ 1939 Hobart 174/14 C dielectric gas. Also discloses special means for mounting and 2,459,612 1/1949 Baker 200/148 B sealing the high-pressure air line with respect to its column 3,179,423 4/1965 McCloud 200/150 X and tank.

.72 J2 32 26 32 It as a as l u l 36 Z6 Z6 7 44 4s U 44 a0 46 Z5 42 Z0/ J6 J7 /00 120 f ""400 71 1 A 1 g sa 0 l l l l- I !\AE .716 A. wi -ii IIII/III)2IIIIII/ I V s PATENTEUUCT 12 I97! sum 1 or 2 INVENTORS JOHN A. OPPEL I Roamr J. fiA/zro/v,

ATTORNEY v PATENTEUnm 12 l97| SHEET 2 OF 2 uvvmrons JOHN A. OPPEL,

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5y Maw 7 ATTORNEY AIRBLAST CIRCUIT BREAKER WI'III IMPROVED SEALING MEANS BACKGROUND This invention relates to an airblast circuit breaker that comprises interrupter tanks at high voltage filled with highpressure air and supported on hollow insulating columns and, more particularly, relates to a circuit breaker of this type that includes high-voltage bushings extending into the tanks and filled with a pressurized dielectric gas other than air. The invention is especially concerned with constructing acircuit breaker of this type in such a manner thata high-reliability seal is maintained between its air system and its dielectric gas system. In a prior form of such circuit breaker, the hollow insulating support columns for the interrupter tanks have contained dielectric gas of the same type as the bushings and have served as reservoirs from which the bushings are supplied with dielectric gas. The high-pressure air contained in each interrupter tank has been supplied to the tank through an insulating air line extending through the interior of the dielectric gas-filled supporting column.

This prior type of circuit breaker construction has presented some difficult problems in maintaining good seals between the two gas systems. Expensive seal constructions, e.g., involving spaced O-rings with a vented space between them, have typically been required, and even these have not always performed as well as might be desired. One source of the problem is that the two media have been in juxtaposition and the O-ring seals for the two systems have been close to one another, increasing the change of mixing of the two media.

SUMMARY An object of the present invention is to construct the circuit breaker in such a manner as to improve the seal between the two media and to make possible the use of simpler seal constructions in certain key locations in the circuit breaker.

Another object is to provide a ring-type seal for the upper end of such an air line which is resistant to damage resulting from sliding caused by temperature-induced expansion and contraction of the airline.

Another object is to reduce the number of locations along the length of the air line where the seals used must accommodate large amounts of sliding motion, thus reducing the likelihood that the seals will be damaged by wear or other forces.

Still another object is to provide an air line mounting arrangement for such a circuit breaker that facilitates replacement of the seals should this become necessary and, more specifically, does not require removal of the interrupter tank to effect such seal replacement.

In carrying out the invention in one form, we provide a plurality of series-connected interrupting assemblies, each comprising an insulating column, a high-voltage interrupter tank mounted atop the insulating column and filled with high-pressure air, and terminal bushings projecting into the tank and containing a dielectric gas other than air. Adjacent one of the interrupting assemblies, there is a freestanding current trans former that comprises a primary circuit and an insulating housing around a portion of the primary circuit. This insulating housing is pneumatically connected to the bushings of the interrupting assembliesby tubing located in the high-voltage region of the circuit breaker. The insulating housing contains a supply of dielectric gas of the same type as the bushings and is thus able to serve as a reservoir from which the bushings are supplied. The hollow insulating columns that support the intubular metal end fittings secured in sealed relationship to opposite ends of the tube. The lower end fitting is secured in sealed relationship to an end plate of the insulating column located at its lower end. Secured to an end plate of the insulating column located at its upper end, there is a metal pipe constituting a portion of the air line and projecting into the interrupter tank. This metal pipe has a bore in which the upper end fitting is slidably received. Ring-type sealing structure is provided between said bore and the outer surface of the upper end fitting to provide a seal therebetween that allows said sliding motion to occur.

BRIEF DESCRIPTION OFF DRAWINGS Forwbetter understanding of the invention, reference may be had to the following description taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a side elevational view partially in section and partially schematic showing a circuit breaker embodying one form of the present invention.

FIG. 2 is a sectional view of a portion of the circuit breaker of FIG. 1, which portion is located at the bottom of the air line.

FIG. 3 is a sectional view of another portion of the circuit breaker of FIG. I, which portion is located at the top of the air line.

FIG. 4 is a more detailed sectional view of a portion of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1, there is shown a high-voltage electric circuit breaker comprising a pair of interrupting assemblies l2 and 14 electrically connected in series in a high-voltage power circuit 15. Since these interrupting assemblies are substantially identical, only one will be described, and identical reference numerals will be used for both assemblies to designate corresponding parts. Assembly 14 comprises a hollow insulating column 16 comprising a tubular casing 17, preferably of porcelain. At its lower end the insulating column 16 is mounted on and attached to a fixed base 18. At its upper end the insulating column 16 carries a metal tank 20 that is filled with compressed air and houses the interrupting elements of interrupter assembly 14. The details of these interrupting elements form no part of the present invention and may be of any suitable conventional form, such as disclosed and claimed, for example, US. Pat. No. 2,783,338-Beatty, assigned to the assignee of the present invention.

It is believed sufficient for the present application to point out that the interrupting elements within tank 20 comprise two sets of relatively movable contacts 24 and 25 electrically connected in series. Each set of contacts comprises a stationary contact 26 and a movable contact 27 mounted for pivotal motion on a stationary pivot 28. When movable contact 27 is pivoted away from is stationary contact 26, an electric arc is formed between the contacts, and this are is soon extinguished in a known manner to interrupt the circuit. The two movable contacts 27 are operated simultaneously by means of an operating mechanism (not shown) within a centrally located metal casing 30. This operating mechanism is preferably of the type shown and claimed in the aforesaid Beatty patent. The two sets of contacts 24 and 25 are electrically connected in series through the metal casing 30. Since metal casing 30 is also connected to tank 20, the tank is at line potential when the circuit breaker is closed.

For extinguishing the are that is fonned when the contacts are separated, a blast valve (not shown) in the tank 20 is opened to direct a portion of the compressed air in the tank through the arcing region into the surrounding atmosphere via the open blast valve. When interruption is completed, the blast valve is closed in order to conserve compressed airin the tank 20. A typical normal pressure for the air in tank 20 is 800 HIGH-PRESSURE AIR LINE For replenishing the supply of compressed air in each of the tanks in order to maintain the tank pressure above a predetermined level, a high-pressure air line 50 is provided for each tank. Each air line 50 extends vertically upward through hollow insulating column 16 from the base of the column into the metal tank 20. At its bottom end, each air line 50 is connected to a high-pressure air supply (not shown) from which high-pressure air is admitted to the air line through a suitable regulator valve (not shown) when the tank pressure falls below said predetermined level. It is to be understood that the air line normally contains high-pressure air at the same pressure as the pressure in tank 20. The major portion 52 of the length of each air line is of a high-strength insulating material such as fiber-glass-reinforced polyester resin. The uppermost portion of the air line is a metal pipe 51 that spans the space between the top of the insulating column 16 and the metal tank 20. The top of metal pipe 51 enters the tank interior through an opening 41 in the lower tank wall, with a suitable O-ring seal 53 being provided between the opening and the outer periphery of pipe 51.

HIGH-VOLGATE BUSHINGS Each of the stationary contacts 26 of the interrupter is mounted on a high-voltage terminal bushing 32 which extends through one end wall of metal tank 20. This high-voltage bushing is of a conventional design, preferably corresponding to that shown and claimed in US. Pat. No. 3,009,983, Oppel, assigned to the assignee of the present invention. The bushing 32 comprises a central conductive stud 34 and an insulating housing concentrically surrounding stud 34. The insulating housing comprises a pair of aligned porcelain shells 36 and 38 and a metal spacing ring 40 therebetween. The parts 36, 38 and 40 are suitably clamped between a pair of shoulders 42 and 44 on stud 34 at opposite ends of the insulating housing. Suitable gaskets (not shown) are provided between the adjacent abutting components of the bushing to seal the interior of the bushing from the surrounding media.

Each tank 20 is provided with two high-voltage bushings 32, and all of these bushings are preferably substantially identical. These bushings serve not only to support the stationary contacts 26 of the interrupters but also to insulate these contacts and studs 34 from tank 20 when the breaker is open. To aid the bushings in preforming their insulating function, each is filled with a high-quality dielectric gas, such as sulfur hexafluoride. The interiors of all of the bushings are placed in communication with each other by metallic tubing 45 and 46. Tubing 45 spans the metal tank 20 of each interrupter assembly and is connected at each end to an opening extending laterally through central spacer ring 40 of one of the bushings. Tubing 46 is connected between the outer ends of the two immediately adjacent bushings of the two interrupter assemblies. The dielectric gas that fills the bushings is supplied to the right-hand bushing 32 of interrupter assembly 14 via a tube 49 at the outer end of the bushing. The tubing 45 and 46 allows the gas entering the tube 49 to fill all the bushings.

CURRENT TRANSFORMER formers will be provided at both sides of the two assemblies. A

preferred form of current transformer is shown and claimed in U.S.Pati. No, 3,456,220 Stewart, assigned to the assignee of the present invention. This current transformer will be described herein only in sufficient detail as to afford an understanding of the present invention. This current transformer comprises a grounded metal tank 62 and a vertically extending hollow insulating housing 64 mounted atop the tank. The primary winding of the transformer comprises two spaced-apart conductive arms 66 and 68, which are mounted in coaxial relationship and are connected in series by means of a loopshaped portion (not shown) of the primary winding located within tank 62. Inner conductor 68 is electrically connected to an external portion of power circuit 15, and outer conductor 66 is electrically connected to a metal plate 70 extending across the top of insulating housing 64. Current entering the current transformer through inner arm 68 flows downwardly through arm 68 through the loop-shaped conductor portion (not shown) located in tank 62 and then upwardly through outer conductor 66 to top metal plate 70. A suitable conductor 72 between top metal plate 70 and stud 34 of the righthand bushing 32 carries current between the current transformer and the interrupter assembly 14. Conducting anns 66 and 68 are locally insulated from each other by means comprising an insulating sleeve 74 near the top of the conductive arms.

UTILIZING THE CURRENT TRANSFORMER AS A GAS- SUPPLY RESERVOIR FOR THE BUSHINGS The interior of current transformer 60 is filled with the same dielectric gas as contained in the bushings 32. In accordance with one aspect of our invention, the current transformer housing serves as a reservoir for supplying the bushings with their dielectric gas filler. To this end, tube 49 communicates with the interior of the current transformer at the top of insulating column 64 and thus pneumatically connects the interior of right-hand bushing 32 with the interior of current transformer 60. Thus, any dielectric gas lost from any of the bushings 32 can be made up by gas supplied from the interior of the current transformer, via tube 49. The volume of free space in the current transformer 60 is very large compared to that in the bushings, e.g., about 25 times that of each bushing, and thus a bushing can be replenished from the current transformer volume with little effect on the pressure of the dielectric gas in the overall system. The pressure in the system is maintained above a predetermined level by supplying gas to the interior of current transformer tank 62 either manually or through a suitable regulator valve (not shown). A typical normal pressure of the dielectric gas is about 50 p.s.i. gauge.

Note that the tubes 45, 46, and49, being mounted atop insulating columns 16 and insulating housing 64, are in regions that are normally at line voltage. Since they extendbetween zones of the same potential, they require no insulating ability and may, if desired, be made of metal. However, to facilitate electrical testing and to preclude significant current flow through the tubing, we prefer to use a suitable electrical insulating material for tubes 46 and 49.

In a typical prior design of this general type circuit breaker, the insulating columns 16 that support the interrupter tanks 20 have been relied upon to serve as reservoirs from which the bushings 32 atop each column are supplied with dielectric gas. But by using the separate freestanding current transformer 60 as the reservoir from which all of the bushings 32 are supplied, as is described hereinabove, I can remove from the support columns 16 all of the special dielectric gas previously present. This is a significant advantage, as will soon appear more clearly.

INSULATING SUPPORT COLUMNS To maintain good insulating properties of the parts that are located within the space 54 inside each column 16, we keep this space filled with dry air at a relatively low pressure compared to that in tanks 20 and air lines 50, e.g., about 50 psi. For confining this low-pressure air within space 54, top and bottom metal plates 56 and 57 are provided at the respective upper and lower ends of insulating casing 17 and are suitably sealed thereto. Using air instead of a special dielectric gas in space 54 is advantageous because should there be any leakage from the air line 50 into the space 54, there is no mixing of the special dielectric gas and the air. To minimize the possibility of such mixing, it has been typical practice to provide between the top plate 56 and the air line complex sealing structure, e.g., double O-ring seals with a vented space therebetween. But we can eliminate the need for such complex sealing structure since there is no special dielectric gas near the air line into which the high-pressure air might leak. We are thus able to provide a simple seal between the air line 50 and top plate 56. This seal, which is schematically shown at 98 in FIG. 1, will soon be described in more detail. A suitable pressure relief valve (not shown) provided for each column 16 limits the pressure rise therein in the remote event of high-pressure air leaking into the column.

Since the air line 50 is primarily of fiber-glass-reinforced polyester resin and the insulating casing 17 is primarily of porcelain, a given temperature change will produce an appreciably different expansion of these two parts. This is accentuated by the fact that typically these are both relatively long parts, e.g., feet or more. To accommodate this differential expansion, we fix the lower end of the air line to the bottom metal plate 57 and allow the top end of the air line to slide relative to top plate 56.

JOINTS BETWEEN AIR LINE AND SUPPORT COLUMN COMPONENTS The joint between the lower end of the air line and the bottom plate 57 is depicted in greater detail in FIG. 2. In this joint, the tubular metal fitting 80 on the lower end of the air line extends through an opening 81 in the end plate and has a peripheral groove 83 of generally V-form therein. A split ring 85 fits into this groove and is bolted to end plate 57. An O-ring 84 between metal fitting 80 and end plate 57 provides a leak proof seal between these parts.

The joint between the upper end of the air line and the top plate 56 is depicted in detail in FIG. 3. In this joint, a tubular metal fitting 90 is bonded to the top of the tubular resin portion 52 of the air line. This metal fitting 90 extends freely through an opening 88 in the top plate 56 and is slidably received in the bore of the metal pipe 51 which extends between top plate 56 and interrupter tank 20. Metal pipe 51 is secured to the upper end plate 56 by means of a split ring 91 fitting into a groove in the outer periphery of the pipe 51 and bearing against the top of end plate 56. A clamping ring 92 bears against an external shoulder 94 on pipe 51 and is clamped to top plate 56 by screws 93. When screws 93 are tightened against the opposition of spring washers 95, they force ring 91 against the top of end plate 56, thereby clamping pipe 51 to plate 56. Pipe 51 has a cylindrical portion fitting into the opening 88 in the top plate 56 and provided with an O-ring 89 that provides a leakproof seal between pipe 51 and top plate 56. The space surrounding pipe 51 is at atmospheric pressure.

If the air line 50 increases its length relative to porcelain casing 17 in response to a temperature change, the metal fitting 90 on the upper end of the air line slides within the bore of metal pipe 51 to accommodate this differential expansion. Sealing structure 98 preferably of the O-ring type is carried in a groove in metal fitting 90 to provide a seal between fitting 90 and the bore of pipe 51 that allows such relative movement without permitting leakage between parts 51 and 90.

A more detailed showing of a preferred form of sealing structure 98 is shown in FIG. 4. This sealing structure comprises an O-ring element 98a, preferably of a silicone resin, and a surrounding sleeve 98b, sometimes referred to as a slipper seal, of a wear-resistant, low-friction material such as polytetrafiuoroethylene. These parts fit in a groove 98c in the outer periphery of the cylindrical fitting 90. When the fitting 90 moves with respect to the pipe 51, the outer surface of the slipper seal 98b slides along the cylindrical bore of he pipe. Since this is a commercially available sealing structure, no further description of its details is believed necessary.

There is very little movement of metal pipe 51 with respect to the wall of tank 20. This is the case because the tank 20 is supported on the top plate 56 by brackets 100 of a metal having substantially the same coefficient of thermal expansion as the metal of pipe 51. Thus, the tank moves vertically in response to temperature changes by substantially the same amount as the upper end of the pipe, and little or no relative movement of these parts occurs, thereby reducing the duty on O-ring seal 53.

Referring 0 FIG. 3, a much greater amount of vertical movement takes place between the metal end fitting 90 and the bore of metal pipe 51, but this can be accommodated without leakage by the simple O-ring type seal 98. The fact that this O-ring seal 98 is disposed between two metal surfaces instead of between one metal and one insulating surface reduces its wear and its likelihood of being otherwise damaged by the appreciable sliding motion present. Our studies indicate that motion of an O-ring type seal on fiber-glass containing resin in the presence of very high fluid pressures is detrimental to the life of the seal, and we therefore eliminate any such motion on fiberglass resin from out construction.

The other O-ring seals used in or around our air line 50 are substantially free of sliding motion and are thus subject to very little wear. In this respect, note that the lower end of air line 50 is fixed with respect to the lower end plate 57, as shown in FIG. 2, and this substantially eliminates wear on O-ring seal 84 present in that location. Note also that the pipe 51 at the upper end of the air line is clamped to the top plate 56, and thus its O-ring seal 89 is substantially free of sliding motion. This substantial freedom from sliding motion and wear contributes to high reliability and long life.

For guiding the long slender air line 50 during its expansion and contraction, a suitable guide plate 102 is provided intermediate its ends. This guide plate has an opening receiving the air line and may be provided with a suitable bearing for imparting added guidance to the air line. Additional guide plates at other locations along the length of the air line may be provided to impart additional guidance for the air line. This guidance helps to reduce the possibility of damage to sliding O-ring seal 98.

REPLACEMENT OF SEALS It is a simple matter to replace any of our O-ring seals should this be necessary. This can be done by first removing split ring at the lower end of the air line and lowering the major portion 52 of the air line to expose the O-ring 84 in the lower joint for replacement. Then the clamping ring 92 of FIG. 3 can be detached from the upper end plate 56, following which pipe 51 can be slid up part way into tank 20 to expose the O-ring seals 89 and 98 for replacement. Note that all of these steps can be preformed without removing the tank 20 from the insulating column 16, which is a distinct advantage in terms of cost and time saving. The interrupting mechanism inside the tank and the pipe 51 are free of obstructions that would block the desired raising of pipe 51 into the tank 20.

While we have shown and described a particular embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the invention in its broader aspects; and we, therefore, intend herein to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A high-voltage electric circuit breaker of the airblast type comprising:

a. a plurality of interrupter tanks at a high voltage, each containing high-pressure air and circuit-interrupting means disposed in said high-pressure air,

b. at least one high-voltage bushing for each tank projecting into said tank and containing an interior space filled with a pressurized gaseous dielectric other than air,

c. a plurality of generally vertically extending hollow insulating columns respectively supporting said tanks, each being filled with air at a pressure lower than the normal pressure in said tanks,

. a plurality of air lines primarily of insulating material for respectively supplying high-pressure air to said interrupter tanks,

e. each of said air lines extending generally vertically through the hollow insulating column on which its associated tank is supported,

. means for sealing the interior of each of said air lines from the interior of its associated insulating column in a manner that allows a portion of said air line to move relative to said insulating column in response to temperature changes,

g. a high-voltage current transformer mounted adjacent one of said supporting columns and comprising a generally vertically extending insulating housing filled with gaseous dielectric of the same type as in said bushings and further comprising a high-voltage primary winding extending through said insulating housing,

h. means located atop said insulating columns and said insulating housing for interconnecting said insulating housing and the interior spaces in said bushings to allow said insulating housing to act as a reservoir from which the gaseous dielectric supply in said bushings can be replenished,

and

. means for electrically connecting in series said interrupting means and said high-voltage primary winding.

The circuit breaker of claim I in which:

. at least one of said insulating columns comprises a tubular casing primarily of insulating material and a pair of end plates respectively secured in sealed relationship to the upper and lower ends of said tubular casing,

b. the air line that extends through said one insulating column comprises a long slender tubular portion of insulating material and a pair of tubular metal fittings secured in sealed relationship to opposite ends of said tubular insulating portion,

c. means is provided for securing the lower one of said end fittings to the lower one of said end plates in sealed relationship thereto,

d. said air line further comprises a metal pipe fixed to said upper end plate and having a bore receiving said upper metal end fitting in sliding relationship and ring-type sealing structure between the outer surface of said metal fitting and said bore for providing a seal therebetween that allows sliding motion of said upper end fitting within said bore,

c. said metal pipe extends from said upper end plate to said tank.

3. The circuit breaker of claim 2 in which:

a. said metal pipe is fixed to said upper end plate by releasable fastening means which allows said metal pipe to be detached from said upper end plate,

b. said metal pipe is slidable within the wall of said tank so that following detachment from said upper end plate said pipe can be raised partially into said tank,

. c. said interrupting means and said pipe are so constructed comprising:

a. an interrupter tank at a high voltage containing high-pressure air and circuit-interrupting means disposed in said high-pressure air,

b. a generally vertically extending hollow insulating column supporting said tank and filled with gas at a pressure lower than the normal pressure in said tank, c. said insulating column compnslng a tubular casing primarily of insulating material and a pair of end plates respectively secured in sealed relationship to the upper and lower ends of said tubular casing,

d. an air line for supplying high-pressure air to said interrupter tank, said air line extending generally vertically through said hollow insulating column,

c. said air line comprising a long slender tubular portion of insulating material and a pair of tubular metal fittings secured in sealed relationship to opposite ends of said insulating material portion,

f. means for securing the lower one of said end fittings to he lower one of said end plates in sealed relationship thereto,

g. said air line further comprising a metal pipe fixed to said upper end plate and having a bore receiving said upper end fitting in slidable relationship,

h. ring-type sealing structure between the outer surface of said metal fitting and said bore for providing a seal therebetween that allows sliding motion of said upper end fitting within said bore when said insulating material portion changes its length in response to temperature changes, and

i. said metal pipe extending from said upper end plate to said tank.

5. The circuit breaker of claim 4 in which:

a. said metal pipe is fixed to said upper end plate by releasable fastening means which allows said metal pipe to be detached from said upper end plate,

b. said metal pipe is slidable within the wall of said tank so that following detachment from said upper end plate said pipe can be released partially into said tank,

c. said interrupting means and said pipe are so constructed that the pipe, when detached from said upper end plate, can be raised sufficiently into said tank to expose said ring-type sealing structure and allow for its replacement without necessitating removal of said tank from said insulating column. 

1. A high-voltage electric circuit breaker of the airblast type comprising: a. a plurality of interrupter tanks at a high voltage, each containing high-pressure air and circuit-interrupting means disposed in said high-pressure air, b. at least one high-voltage bushing for each tank projecting into said tank and containing an interior space filled with a pressurized gaseous dielectric other than air, c. a plurality of generally vertically extending hollow insulating columns respectively supporting said tanks, each being filled with air at a pressure lower than the normal pressure in said tanks, d. a plurality of air lines primarily of insulating material for respectively supplying high-pressure air to said interrupter tanks, e. each of said air lines extending generally vertically through the hollow insulating column on which its associated tank is supported, f. means for sealing the interior of each of said air lines from the interior of its associated insulating column in a manner that allows a portion of said air line to move relative to said insulating column in response to temperature changes, g. a high-voltage current transformer mounted adjacent one of said supporting columns and comprising a generally vertically extending insulating housing filled with gaseous dielectric of the same type as in said bushings and further comprising a high-voltage primary winding extending through said insulating housing, h. means located atop said insulating columns and said insulating housing for interconnecting said insulating housing and the interior spaces in said bushings to allow said insulating housing to act as a reservoir from which the gaseous dielectric supply in said bushings can be replenished, and i. means for electrically connecting in series said interrupting means and said high-voltage primary winding.
 2. The circuit breaker of claim 1 in which: a. at least one of said insulating columns comprises a tubular casing primarily of insulating material and a pair of end plates respectively secured in sealed relationship to the upper and lower ends of said tubular casing, b. the air line that extends through said one insulating column comprises a long slender tubular portion of insulating material and a pair of tubular metal fittings secured in sealed relationship to opposite ends of said tubular insulating portion, c. means is provided for securing the lower one of said end fittings to the lower one of said end plates in sealed relationship thereto, d. said air line further comprises a metal pipe fixed to said upper end plate and having a bore receiving said upper metal end fitting in sliding relationship and ring-type sealing structure between the outer surface of saiD metal fitting and said bore for providing a seal therebetween that allows sliding motion of said upper end fitting within said bore, e. said metal pipe extends from said upper end plate to said tank.
 3. The circuit breaker of claim 2 in which: a. said metal pipe is fixed to said upper end plate by releasable fastening means which allows said metal pipe to be detached from said upper end plate, b. said metal pipe is slidable within the wall of said tank so that following detachment from said upper end plate said pipe can be raised partially into said tank, c. said interrupting means and said pipe are so constructed that the pipe, when detached from said upper end plate, can be raised sufficiently into said tank to expose said ring-type sealing structure and allow for its replacement without necessitating removal of said tank from said insulating column.
 4. A high-voltage electric circuit breaker of the airblast type comprising: a. an interrupter tank at a high voltage containing high-pressure air and circuit-interrupting means disposed in said high-pressure air, b. a generally vertically extending hollow insulating column supporting said tank and filled with gas at a pressure lower than the normal pressure in said tank, c. said insulating column comprising a tubular casing primarily of insulating material and a pair of end plates respectively secured in sealed relationship to the upper and lower ends of said tubular casing, d. an air line for supplying high-pressure air to said interrupter tank, said air line extending generally vertically through said hollow insulating column, e. said air line comprising a long slender tubular portion of insulating material and a pair of tubular metal fittings secured in sealed relationship to opposite ends of said insulating material portion, f. means for securing the lower one of said end fittings to he lower one of said end plates in sealed relationship thereto, g. said air line further comprising a metal pipe fixed to said upper end plate and having a bore receiving said upper end fitting in slidable relationship, h. ring-type sealing structure between the outer surface of said metal fitting and said bore for providing a seal therebetween that allows sliding motion of said upper end fitting within said bore when said insulating material portion changes its length in response to temperature changes, and i. said metal pipe extending from said upper end plate to said tank.
 5. The circuit breaker of claim 4 in which: a. said metal pipe is fixed to said upper end plate by releasable fastening means which allows said metal pipe to be detached from said upper end plate, b. said metal pipe is slidable within the wall of said tank so that following detachment from said upper end plate said pipe can be released partially into said tank, c. said interrupting means and said pipe are so constructed that the pipe, when detached from said upper end plate, can be raised sufficiently into said tank to expose said ring-type sealing structure and allow for its replacement without necessitating removal of said tank from said insulating column. 